Exhaust gas purification systems of the abovementioned type are known for motor vehicles driven by diesel engines, which have what is known as an SCR catalytic converter for removing nitrogen oxides from the exhaust gas. This allows the reduction of nitrogen oxides from the exhaust gas fed to it even under oxidizing conditions, i.e., with an exhaust gas which has an excess of oxygen. However, a precondition for these systems is that a reducing agent, which has a selective activity with regard to the reduction of nitrogen oxides, must be added to the exhaust gas. Such reducing agents include, inter alia, primarily ammonia and substances which can release ammonia. In particular, urea or ammonium carbamate are customary and are preferably carried in a corresponding tank in the motor vehicle, preferably in the form of an aqueous solution. This reducing agent therefore constitutes an additional operating fluid which has to be refilled at regular intervals to ensure the long-term exhaust gas purification function. This represents additional outlay and requires careful handling to minimize associated risks, for example risks caused by incorrect usage of the reducing agent.